Composer Reference

The composer Node.js module makes it possible define action compositions using combinators and deploy them.

Installation

To install the composer module use the Node Package Manager:

npm -g install @ibm-functions/composer

We recommend to install the module globally (with -g option) so the compose command is added to the path. Otherwise, it can be found in the bin folder of the module installation.

Example

A composition is typically defined by means of a Javascript file as illustrated in samples/demo.js:

composer.if(
    composer.action('authenticate', { action: function main({ password }) { return { value: password === 'abc123' } } }),
    composer.action('success', { action: function main() { return { message: 'success' } } }),
    composer.action('failure', { action: function main() { return { message: 'failure' } } }))

Composer offers traditional control-flow concepts as methods. These methods are called combinators. This example composition composes three actions named authenticate, success, and failure using the composer.if combinator, which implements the usual conditional construct. It take three actions (or compositions) as parameters. It invokes the first one and, depending on the result of this invocation, invokes either the second or third action.

This composition includes the definitions of the three composed actions. If the actions are defined and deployed elsewhere, the composition code can be shorten to:

composer.if('authenticate', 'success', 'failure')

To deploy this composition use the compose command:

compose demo.js --deploy demo

The compose command synthesizes and deploy an action named demo that implements the composition. It also deploys the composed actions if definitions are provided for them.

The demo composition may be invoked like any action, for instance using the OpenWhisk CLI:

wsk action invoke demo -r -p password passw0rd

{
    "message": "failure"
}

Activation Records

An invocation of a composition creates a series of activation records:

wsk action invoke demo -p password passw0rd

ok: invoked /_/demo with id 4f91f9ed0d874aaa91f9ed0d87baaa07

wsk activation list

activations
fd89b99a90a1462a89b99a90a1d62a8e demo
eaec119273d94087ac119273d90087d0 failure
3624ad829d4044afa4ad829d40e4af60 demo
a1f58ade9b1e4c26b58ade9b1e4c2614 authenticate
3624ad829d4044afa4ad829d40e4af60 demo
4f91f9ed0d874aaa91f9ed0d87baaa07 demo

The entry with the earliest start time (4f91f9ed0d874aaa91f9ed0d87baaa07) summarizes the invocation of the composition while other entries record later activations caused by the composition invocation. There is one entry for each invocation of a composed action (a1f58ade9b1e4c26b58ade9b1e4c2614 and eaec119273d94087ac119273d90087d0). The remaining entries record the beginning and end of the composition as well as the transitions between the composed actions.

Compositions are implemented by means of OpenWhisk conductor actions. The documentation of conductor actions discusses activation records in greater details.

Deployment

The compose command when not invoked with the --deploy option returns the composition encoded as a JSON dictionary:

compose demo.js

{
    "actions": [
        {
            "name": "/_/authenticate",
            "action": {
                "exec": {
                    "kind": "nodejs:default",
                    "code": "function main({ password }) { return { value: password === 'abc123' } }"
                }
            }
        },
        {
            "name": "/_/success",
            "action": {
                "exec": {
                    "kind": "nodejs:default",
                    "code": "function main() { return { message: 'success' } }"
                }
            }
        },
        {
            "name": "/_/failure",
            "action": {
                "exec": {
                    "kind": "nodejs:default",
                    "code": "function main() { return { message: 'failure' } }"
                }
            }
        }
    ],
    "composition": [
        {
            "type": "if",
            "test": [
                {
                    "type": "action",
                    "name": "/_/authenticate"
                }
            ],
            "consequent": [
                {
                    "type": "action",
                    "name": "/_/success"
                }
            ],
            "alternate": [
                {
                    "type": "action",
                    "name": "/_/failure"
                }
            ]
        }
    ]
}

The JSON format is documented in FORMAT.md. The format is meant to be stable, self-contained, language-independent, and human-readable. The JSON dictionary includes the definition for the composition as well as definitions of nested actions and compositions (if any).

A JSON-encoded composition may be deployed using the compose command:

compose demo.js > demo.json
compose demo.json --deploy demo

The compose command can also produce the code of the conductor action generated for the composition:

compose demo.js --encode

const main=(function init(e,t){function r(e,t){return e.slice(-1)[0].next=1,e.push(...t),e}const a=function e(t,a=""){if(Array.isArray(t))return 0===t.length?[{type:"pass",path:a}]:t.map((t,r)=>e(t,a+"["+r+"]")).reduce(r);const n=t.options||{};switch(t.type){case"action":return[{type:"action",name:t.name,path:a}];case"function":return[{type:"function",exec:t.exec,path:a}];case"literal":return[{type:"literal",value:t.value,path:a}];case"finally":var s=e(t.body,a+".body");const l=e(t.finalizer,a+".finalizer");return(o=[[{type:"try",path:a}],s,[{type:"exit",path:a}],l].reduce(r))[0].catch=o.length-l.length,o;case"let":return s=e(t.body,a+".body"),[[{type:"let",let:t.declarations,path:a}],s,[{type:"exit",path:a}]].reduce(r);case"retain":s=e(t.body,a+".body");var o=[[{type:"push",path:a}],s,[{type:"pop",collect:!0,path:a}]].reduce(r);return n.field&&(o[0].field=n.field),o;case"try":s=e(t.body,a+".body");const h=r(e(t.handler,a+".handler"),[{type:"pass",path:a}]);return(o=[[{type:"try",path:a}],s].reduce(r))[0].catch=o.length,o.slice(-1)[0].next=h.length,o.push(...h),o;case"if":var p=e(t.consequent,a+".consequent"),c=r(e(t.alternate,a+".alternate"),[{type:"pass",path:a}]);return n.nosave||(p=r([{type:"pop",path:a}],p)),n.nosave||(c=r([{type:"pop",path:a}],c)),o=r(e(t.test,a+".test"),[{type:"choice",then:1,else:p.length+1,path:a}]),n.nosave||(o=r([{type:"push",path:a}],o)),p.slice(-1)[0].next=c.length,o.push(...p),o.push(...c),o;case"while":return p=e(t.body,a+".body"),c=[{type:"pass",path:a}],n.nosave||(p=r([{type:"pop",path:a}],p)),n.nosave||(c=r([{type:"pop",path:a}],c)),o=r(e(t.test,a+".test"),[{type:"choice",then:1,else:p.length+1,path:a}]),n.nosave||(o=r([{type:"push",path:a}],o)),p.slice(-1)[0].next=1-o.length-p.length,o.push(...p),o.push(...c),o;case"dowhile":var i=e(t.test,a+".test");return n.nosave||(i=r([{type:"push",path:a}],i)),o=[e(t.body,a+".body"),i,[{type:"choice",then:1,else:2,path:a}]].reduce(r),n.nosave?(o.slice(-1)[0].then=1-o.length,o.slice(-1)[0].else=1):(o.push({type:"pop",path:a}),o.slice(-1)[0].next=1-o.length),c=[{type:"pass",path:a}],n.nosave||(c=r([{type:"pop",path:a}],c)),o.push(...c),o}}(t),n=e=>"object"==typeof e&&null!==e&&!Array.isArray(e),s=e=>Promise.reject({code:400,error:e}),o=e=>Promise.reject((e=>({code:"number"==typeof e.code&&e.code||500,error:"string"==typeof e.error&&e.error||e.message||"string"==typeof e&&e||"An internal error occurred"}))(e));return t=>Promise.resolve().then(()=>(function(t){let r=0,p=[];if(void 0!==t.$resume){if(!n(t.$resume))return s("The type of optional $resume parameter must be object");if(r=t.$resume.state,p=t.$resume.stack,void 0!==r&&"number"!=typeof r)return s("The type of optional $resume.state parameter must be number");if(!Array.isArray(p))return s("The type of $resume.stack must be an array");delete t.$resume,c()}function c(){if(n(t)||(t={value:t}),void 0!==t.error)for(t={error:t.error},r=void 0;p.length>0&&"number"!=typeof(r=p.shift().catch););}function i(r){function a(e,t){const r=p.find(t=>void 0!==t.let&&void 0!==t.let[e]);void 0!==r&&(r.let[e]=JSON.parse(JSON.stringify(t)))}const n=p.reduceRight((e,t)=>"object"==typeof t.let?Object.assign(e,t.let):e,{});let s="(function(){try{";for(const e in n)s+=`var ${e}=arguments[1]['${e}'];`;s+=`return eval((${r}))(arguments[0])}finally{`;for(const e in n)s+=`arguments[1]['${e}']=${e};`;s+="}})";try{return e(s)(t,n)}finally{for(const e in n)a(e,n[e])}}for(;;){if(void 0===r)return console.log("Entering final state"),console.log(JSON.stringify(t)),t.error?t:{params:t};const e=a[r];console.log(`Entering state ${r} at path fsm${e.path}`);const n=r;switch(r=void 0===e.next?void 0:n+e.next,e.type){case"choice":r=n+(t.value?e.then:e.else);break;case"try":p.unshift({catch:n+e.catch});break;case"let":p.unshift({let:JSON.parse(JSON.stringify(e.let))});break;case"exit":if(0===p.length)return o(`State ${n} attempted to pop from an empty stack`);p.shift();break;case"push":p.unshift(JSON.parse(JSON.stringify({params:e.field?t[e.field]:t})));break;case"pop":if(0===p.length)return o(`State ${n} attempted to pop from an empty stack`);t=e.collect?{params:p.shift().params,result:t}:p.shift().params;break;case"action":return{action:e.name,params:t,state:{$resume:{state:r,stack:p}}};case"literal":t=JSON.parse(JSON.stringify(e.value)),c();break;case"function":let a;try{a=i(e.exec.code)}catch(e){console.error(e),a={error:`An exception was caught at state ${n} (see log for details)`}}"function"==typeof a&&(a={error:`State ${n} evaluated to a function`}),t=JSON.parse(JSON.stringify(void 0===a?t:a)),c();break;case"pass":c();break;default:return o(`State ${n} has an unknown type`)}}})(t)).catch(o)})(eval,[{"type":"if","test":[{"type":"action","name":"/_/authenticate"}],"consequent":[{"type":"action","name":"/_/success"}],"alternate":[{"type":"action","name":"/_/failure"}]}])

This code may be deployed using the OpenWhisk CLI:

compose demo.js > demo-conductor.js
wsk action create demo demo-conductor.js -a conductor true

In contrast to the JSON format, the conductor action code does not include definitions for nested actions or compositions.

Parameter Objects and Error Objects

A composition, like any action, accepts a JSON dictionary (the input parameter object) and produces a JSON dictionary (the output parameter object). An output parameter object with an error field is an error object. A composition fails if it produces an error object.

By convention, an error object returned by a composition is stripped from all fields except from the error field. This behavior is consistent with the OpenWhisk action semantics, e.g., the action with code function main() { return { error: 'KO', message: 'OK' } } outputs { error: 'KO' }.

Combinators

The composer module offers a number of combinators to define compositions:

The action, function, and literal combinators and their synonymous construct compositions respectively from actions, functions, and constant values. The other combinators combine existing compositions to produce new compositions.

Where a composition is expected, the following shorthands are permitted:

• name of type string stands for composer.action(name),
• fun of type function stands for composer.function(fun),
• null stands for the empty sequence composer.sequence().

Action

composer.action(name, [options]) is a composition with a single action named name. It invokes the action named name on the input parameter object for the composition and returns the output parameter object of this action invocation.

The action name may specify the namespace and/or package containing the action following the usual OpenWhisk grammar. If no namespace is specified, the default namespace is assumed. If no package is specified, the default package is assumed.

Examples:

composer.action('hello')
composer.action('myPackage/myAction')
composer.action('/whisk.system/utils/echo')

The optional options dictionary makes it possible to provide a definition for the action being composed:

// specify the code for the action
composer.action('hello', { action: function main() { return { message: 'hello' } } })
composer.action('hello', { action: "function main() { return { message: 'hello' } }" })
composer.action('hello', {
    action: {
        kind: 'nodejs:default',
        code: "function main() { return { message: 'hello' } }"
    }
})

// specify a file containing the code for the action
composer.action('hello', { filename: 'hello.js' })

// define an action sequence
composer.action('helloAndBye', { sequence: ['hello', 'bye'] })

Function

composer.function(fun) is a composition with a single Javascript function fun. It applies the specified function to the input parameter object for the composition.

• If the function returns a value of type function, the composition returns an error object.
• If the function throws an exception, the composition returns an error object. The exception is logged as part of the conductor action invocation.
• If the function returns a value of type other than function, the value is first converted to a JSON value using JSON.stringify followed by JSON.parse. If the resulting JSON value is not a JSON dictionary, the JSON value is then wrapped into a { value } dictionary. The composition returns the final JSON dictionary.
• If the function does not return a value and does not throw an exception, the composition returns the input parameter object for the composition converted to a JSON dictionary using JSON.stringify followed by JSON.parse.

Examples:

composer.function(params => ({ message: 'Hello ' + params.name }))
composer.function(function (params) { return { error: 'error' } })

function product({ x, y }) { return { product: x * y } }
composer.function(product)

Environment capture

Functions intended for compositions cannot capture any part of their declaration environment. They may however access and mutate variables in an environment consisting of the variables declared by the composer.let combinator discussed below.

The following is not legal:

let name = 'Dave'
composer.function(params => ({ message: 'Hello ' + name }))

The following is legal:

composer.let({ name: 'Dave' }, composer.function(params => ({ message: 'Hello ' + name })))

Literal

composer.literal(value) and its synonymous composer.value(value) output a constant JSON dictionary. This dictionary is obtained by first converting the value argument to JSON using JSON.stringify followed by JSON.parse. If the resulting JSON value is not a JSON dictionary, the JSON value is then wrapped into a { value } dictionary.

The value argument may be computed at composition time. For instance, the following composition captures the date at the time the composition is encoded to JSON:

composer.literal(Date())

Sequence

composer.sequence(composition_1, composition_2, ...) chains a series of compositions (possibly empty).

The input parameter object for the composition is the input parameter object of the first composition in the sequence. The output parameter object of one composition in the sequence is the input parameter object for the next composition in the sequence. The output parameter object of the last composition in the sequence is the output parameter object for the composition.

If one of the components fails, the remainder of the sequence is not executed. The output parameter object for the composition is the error object produced by the failed component.

An empty sequence behaves as a sequence with a single function params => params. The output parameter object for the empty sequence is its input parameter object unless it is an error object, in which case, as usual, the error object only contains the error field of the input parameter object.

Let

composer.let({ name_1: value_1, name_2: value_2, ... }, composition_1_, _composition_2_, ...) declares one or more variables with the given names and initial values, and runs runs a sequence of compositions in the scope of these declarations.

Variables declared with composer.let may be accessed and mutated by functions running as part of the following sequence (irrespective of their place of definition). In other words, name resolution is dynamic. If a variable declaration is nested inside a declaration of a variable with the same name, the innermost declaration masks the earlier declarations.

For example, the following composition invokes composition composition repeatedly n times.

composer.let({ i: n }, composer.while(() => i-- > 0, composition))

Variables declared with composer.let are not visible to invoked actions. However, they may be passed as parameters to actions as for instance in:

composer.let({ n: 42 }, () => ({ n }), 'increment', params => { n = params.n })

In this example, the variable n is exposed to the invoked action as a field of the input parameter object. Moreover, the value of the field n of the output parameter object is assigned back to variable n.

If

composer.if(condition, consequent, [alternate], [options]) runs either the consequent composition if the condition evaluates to true or the alternate composition if not.

A condition composition evaluates to true if and only if it produces a JSON dictionary with a field value with value true. Other fields are ignored. Because JSON values other than dictionaries are implicitly lifted to dictionaries with a value field, condition may be a Javascript function returning a Boolean value. An expression such as params.n > 0 is not a valid condition (or in general a valid composition). One should write instead params => params.n > 0. The input parameter object for the composition is the input parameter object for the condition composition.

The alternate composition may be omitted. If condition fails, neither branch is executed.

The optional options dictionary supports a nosave option. If options.nosave is thruthy, the consequent composition or alternate composition is invoked on the output parameter object of the condition composition. Otherwise, the output parameter object of the condition composition is discarded and the consequent composition or alternate composition is invoked on the input parameter object for the composition. For example, the following compositions divide parameter n by two if n is even:

composer.if(params => params.n % 2 === 0, params => { params.n /= 2 })
composer.if(params => { params.value = params.n % 2 === 0 }, params => { params.n /= 2 }, null, { nosave: true })

In the first example, the condition function simply returns a Boolean value. The consequent function uses the saved input parameter object to compute n's value. In the second example, the condition function adds a value field to the input parameter object. The consequent function applies to the resulting object. In particular, in the second example, the output parameter object for the condition includes the value field.

While, the default nosave == false behavior is typically more convenient, preserving the input parameter object is not free as it counts toward the parameter size limit for OpenWhisk actions. In essence, the limit on the size of parameter objects processed during the evaluation of the condition is reduced by the size of the saved parameter object. The nosave option omits the parameter save, hence preserving the parameter size limit.

While

composer.while(condition, body, [options]) runs body repeatedly while condition evaluates to true. The condition composition is evaluated before any execution of the body composition. See composer.if for a discussion of conditions.

A failure of condition or body interrupts the execution. The composition returns the error object from the failed component.

Like composer.if, composer.while supports a nosave option. By default, the output parameter object of the condition composition is discarded and the input parameter object for the body composition is either the input parameter object for the whole composition the first time around or the output parameter object of the previous iteration of body. However if options.nosave is thruthy, the input parameter object for body is the output parameter object of condition. Moreover, the output parameter object for the whole composition is the output parameter object of the last condition evaluation.

For instance, the following composition invoked on dictionary { n: 28 } outputs { n: 7 }:

composer.while(params => params.n % 2 === 0, params => { params.n /= 2 })

For instance, the following composition invoked on dictionary { n: 28 } outputs { n: 7, value: false }:

composer.while(params => { params.value = params.n % 2 === 0 }, params => { params.n /= 2 }, { nosave: true })

Dowhile

composer.dowhile(condition, body, [options]) is similar to composer.while(body, condition, [options]) except that body is invoked before condition is evaluated, hence body is always invoked at least once.

Repeat

composer.repeat(count, body) invokes body count times.

Try

composer.try(body, handler) runs body with error handler handler.

If body outputs an error object, handler is invoked with this error object as its input parameter object. Otherwise, handler is not run.

Finally

composer.finally(body, finalizer) runs body and then finalizer.

The finalizer is invoked in sequence after body even if body returns an error object.

Retry

composer.retry(count, body) runs body and retries body up to count times if it fails. The output parameter object for the composition is either the output parameter object of the successful body invocation or the error object produced by the last body invocation.

Retain

composer.retain(body, [options]) runs body on the input parameter object producing an object with two fields params and result such that params is the input parameter object of the composition and result is the output parameter object of body.

An options dictionary object may be specified to alter the default behavior of composer.retain in the following ways:

• If options.catch is thruthy, the retain combinator behavior will be the same even if body returns an error object. Otherwise, if body fails, the output of the retain combinator is only the error object (i.e., the input parameter object is not preserved).
• If options.filter is a function, the combinator only persists the result of the function application to the input parameter object.
• If options.field is a string, the combinator only persists the value of the field of the input parameter object with the given name.